The main password is your owner key - it let's you do anything so it is best to only use the key that you need and avoid using the OK.  Posting lets you post and vote.  Active lets you do transactions - memo let's you send messages but it isn't really used yet.  Only the Owner Key (Main password) allows you to change the other keys and also change the master password itself.  Not sure what WIF stands for perhaps someone else can clarify that.